This application is a continuation application filed under 35 U.S.C. 111 (a) claiming the benefit under 35 U.S.C. 120 and 365 (c) of a PCT International Application No. PCT/JP2003/000285 filed Jan. 15, 2003, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
The PCT International Application No. PCT/JP2003/000285 was published in the English language on Jul. 29, 2004 under International Publication Number WO 2004/064047 A1.
1. Field of the Invention
The present invention generally relates to magnetic recording media and magnetic storage apparatuses, and more particularly to a magnetic recording medium having a seed layer and/or an underlayer made of AlV or an alloy thereof, and to a magnetic storage apparatus which uses such a magnetic recording medium.
2. Description of the Related Art
A typical longitudinal magnetic recording medium, such as a magnetic disk, is provided with a substrate, a seed layer, a Cr or Cr alloy underlayer, a CoCr alloy intermediate layer, a Co alloy magnetic layer where the information is written, a C overlayer, and an organic lubricant which are stacked in this order. Substrates that are being presently used include NiP-plated AlMg and Glass. The latter substrate has been getting more popularity due to its resistance to shock, smoothness, hardness, light weight, and minimum flutter especially at the disk edge.
The microstructure of the magnetic layer, which includes grain size, grain size distribution, preferred orientation and Cr segregation, strongly affects the recording characteristics of the magnetic recording medium. The microstructure of the magnetic layer has widely been controlled by the use of seed layers and underlayers and with suitable mechanical texturing of the substrates. Small grain size and grain size distribution with excellent crystallographic orientation are desired for extending the longitudinal technology for the current densities on the order of 50 Gbits/in2 and for future recording technologies for the densities on the order of 100 Gbits/in2 or greater.
In this specification, the seed layer is defined as a layer which is closest to the substrate and aid primarily in promoting a desired crystallographic orientation on the succeeding layers such as the underlayer deposited thereon. The seed layer is amorphous such as the widely used NiP or B2 structured materials, and has a (002), (110), or (112) fiber texture.
In addition, in this specification, the underlayer is defined as a layer which is grown on the substrate or suitable seed layer and aid primarily in improving the preferred crystallographic orientation for the subsequent deposited intermediate layers and magnetic layers on top of the underlayer. The underlayer is crystalline, as the case of the bcc structured materials such as Cr, and has a (002), (110), or (112) fiber texture. The most extensively used underlayer has been Cr or alloys of Cr with Mo, Mn, V, Ti and W where, typically, the Cr content is at least 70 at. % and the additives are used for increasing the lattice parameter. This lattice parameter increase helps to reduce the lattice mismatch between the Cr underlayer and the Co magnetic layer. These layers are usually deposited on mechanically textured or non-textured Ni81P19 substrates like Glass or Al. Mechanical texturing invariably exposes NiP to air which oxidizes the film surface. Oxidation is important for the Cr to grow with a (002) in plane texture which results in the subsequently deposited magnetic layer to have a (11 20) crystallographic texture.
This is taken advantage of in a U.S. Pat. No. 5,866,227 to Chen et al. which describes a reactively sputtered NiP (with O2) seed layer on Glass substrates. Typically, Cr is deposited at a temperature Ts which is greater than 180° C. to promote a (002) texture with no (110) peak in the XRD spectrum. Low Ts deposition may result in smaller grains but a (110) texture is developed. NiP does not adhere very well to Glass such that an adhesive layer described in a U.S. Pat. No. 6,139,981 to Chuang et al. may be used. On NiP seed layers, underlayer grain sizes in the order of 8 nm to 10 nm can be realized by using two Cr alloy layers and by reducing the total underlayer thickness to less than 10 nm. Increasing the total thickness tends to significantly increase the average grain size. For example, for a single layer of Cr80Mo20, at t=30 nm, the average grain size can be approximately 20 nm which is obviously inadequate for present day media noise requirements.
L. Tang et al. “Microstructure and texture evolution of Cr thin films with thickness”, J. Appl. Phys., vol.74, pp.5025-5032, 1993 also observed grain diameter increase with thickness. To achieve an average grain size less than 8 nm is difficult as further reduction of the underlayer thickness results in degradation of the magnetic layer c-axis in-plane orientation (IPO). Although the underlayer average grain size can be small, a few large grains occasionally occur on which two or more magnetic grains may grow. The effective magnetic anisotropy of such grains may be reduced if magnetic isolation is not complete. Alternate approaches to reduce the grain size include inclusion of B onto the CoCrPt matrix. B inclusion reduces the grain size of recording layer and gives substantial improvement of the media noise and magnetic properties. However, adding very high percentage of B increases the fct phase and hence the crystallographic quality goes bad beyond a certain B percentage, especially over a B concentration of 8%.
A U.S. Pat. No. 5,693,426 to Lee et al. describes ordered intermetallic underlayers with the B2 structure such as NiAl and FeAl. Ordered intermetallic alloys with structures such as B2, L10, and L12 are expected to have small grain sizes presumably due to the strong binding between the component atoms. Both NiAl and FeAl grow on Glass substrates with a (211) fiber texture which makes the magnetic layer c-axis to be in-plane with a (1010) texture. Grain sizes on the order of 12 nm can be achieved even for thick layers greater than 60 nm. The use of both NiAl and Cr on NiP has also been described by a U.S. Pat. No. 6,010,795 to Chen et al. In this case, NiAl develops a (001) texture due to the (002) texture of the crystalline Cr pre-underlayer and the magnetic layer texture is Co(11 20).
There are other seed layers aside from NiP that promote the Cr(002) texture. A U.S. Pat. No. 5,685,958 to Bian et al. describes refractory metals such as Ta, Cr, Nb, W, and Mo with a reactive element consisting of at least 1% nitrogen or oxygen. In the case of Ta, which is reactively sputtered with Ar+N2 gas, as the N2 volume fraction is increased, Cr (002) appears in the XRD spectrum as well as Co(11 20). A typical underlayer thickness of 50 nm was mentioned and wide variations in the thickness were claimed to only slightly affect the media magnetic characteristics. As the volume fraction is increased to 3.3%, both peaks disappear indicating the degradation of crystallographic orientation. Bian et al. proposed a useful range of substrate temperature Ts of 150° C. to 330° C. and a more preferred range of 210° C. to 250° C. This would make the substrate temperature Ts necessary for the deposition of the Cr onto TaN similar to that onto NiP. A useful range of nitrogen partial pressure of 0.1 mTorr to 2 mTorr was also proposed. The nitrogen concentration of the TaN films are unknown but may be between 10 at. % to 50 at. %.
Kataoka et al., “Magnetic and recording characteristics of Cr, Ta, W and Zr pre-coated Glass disks”, IEEE Trans. Magn., vol.31, pp.2734-2736, 1995 has reported Cr, Ta, W, and Zr pre-coating layers on Glass. For Ta films, reactive sputtering with the proper amount of N2 actually improves the succeeding Cr underlayer crystallographic orientation. Cr directly deposited on Glass develop not only the preferred (002) orientation but also an undesirable (110) texture.
Oh et al., “A study on VMn underlayer in CoCrPt Longitudinal Media”, IEEE Trans. Magn., vol.37, pp.1504-1507, 2001 reported a VMn alloy underlayer, where the V content is 71.3 at. % and Mn content is 28.7 at. %. V has a high melting point (1500° C.) and in principle may grow with small grains when sputtered but the texture is a very strong (110) on Glass and on most seed layers. The U.S. Pat. No. 5,693,426 to Lee et al. also proposed polycrystalline seed layers such as MgO (B1 structured) and a myriad of B2 materials such as NiAl and FeAl which form templates for the succeeding Mn-containing alloys.
Good IPO leads to an increase in remanent magnetization and signal thermal stability. It also improves resolution or the capacity of the magnetic recording medium to support high density bits. Recently developed synthetic ferrimagnetic media (SFM) provide improved thermal stability and resolution compared to conventional magnetic recording media of the same Mrt (remanent magnetization and thickness product). Seed layers that can be used for conventional magnetic recording media can also be used for SFM but the potential of the SFM for extending the limits of longitudinal recording can best be realized if the IPO is close to perfect. The IPO can be quantified by low incident angle XRD such as that made by Doerner et al., “Mirostructure and Thermal Stability of Advanced Longitudinal Media”, IEEE Trans. Magn., vol.36, p.43-47, January 2001 and Doerner et al., “Demonstration of 35 Gbits/in2 in media on Glass substrates”, IEEE Trans. Magn., vol.37, pp.1052-1058, March 2001 (10 Gbits/in2 and 35 Gbits/in2 demo) or more simply by taking the ratio of the coercivity normal to and along the film plane (h=Hc⊥/Hc//).
The ratio h for the magnetic recording media on Cr(002)/NiP is typically 0.2<h≦0.15 and is observed only for badly matched underlayers and magnetic layers. For h≦0.15, the M(H) hysteresis loop perpendicular to the film normal is approximately linear with field and Hc⊥ is typically <500 Oe. In the case of NiAl, the (211) texture is weak and a thickness greater than 50 nm is usually needed to realize the above ratio h and to reduce the occurrence of magnetic grains with a (0002) orientation. Previous work on using NiAl directly on Glass as a seed layer for conventional magnetic recording media resulted in poor squareness (h>0.25) and could not match the performance of magnetic recording media on Cr(002)/NiP. This is the case even when seed layers such as NiP and CoCrZr are employed. XRD measurements by Doerner et al. showed that the magnetic c-axes are spread over an angle greater than ±20° compared to less than ±5° for magnetic recording media on NiP/AlMg substrates. For magnetic recording media on TaN, though the Cr(002) and Co(11 20) peaks are visible from the XRD data, h>0.2 and the magnetic recording media underperforms magnetic recording media on Cr(002)/NiP. The Cr alloy underlayer thickness used here is less than 10 nm, and the reduction of h was not observed by further increases in the underlayer thickness to greater than 20 nm. But unlike B2 materials and alloys such as VMn, the average grain diameter of Cr alloy underlayers rapidly increases with thickness.
Aside from the IPO, another concern in the manufacturing of the SFM is the increase in the number of chambers necessary compared to conventional magnetic recording media especially when bare Glass substrates are used. Moreover, as the throughput has to be maintained at a high level, the thickness of the deposited film is typically limited to 30 nm. Seed layers or underlayers that need to be thicker require two chambers. The typical sequential deposition must also be made in a rapid fashion not only to have a high yield but also to prevent the temperature of the high emissivity Glass disk to drop before the magnetic layers are deposited. Else, a heating step is needed which will require a separate process chamber. The disk emissivity is decreased by the seed layer and the underlayer such that both cannot be very thin. If a bias voltage is to be applied as in CVD C deposition, the total magnetic recording medium thickness needed is usually greater than 30 nm.